Moreover, it would leave Neil Lennon's side anchored to the foot of Group H a third of the way through the campaign.
But for Celtic, home is definitely where the heart is strongest, and upsetting the odds has always been in their DNA.
Even for new Bhoys, like Virgil van Dijk, who has helped them keep clean sheets in all three Champions League qualifiers and play-off games at Celtic Park this season, the importance of defending the reputation of Fortress Parkhead is understood.
The giant Dutchman said: "We have to get a result, and a draw would count as one. I hope it is possible."
Of course, replicating the 2-1 win Celtic inflicted on Barca back in November would be even more acceptable.
It would also be the biggest result in the career of the man who joined in the summer from FC Groningen, and would propel van Dijk from an enthralled spectator to starring role.
He explained: "When Celtic beat Barcelona last season, I was watching live on the couch.
"The atmosphere was fantastic. It was unbelievable, even on the TV, and having the fans behind you for every minute and every second from the beginning of the game is so important.
"It can help you a lot when you are not in the game, and that was the biggest thing for me, as well as the result, which was incredible."
It was a result which sent shock waves around the football world.
But the 22-year-old has been in the game long enough to know that, if the spirit and discipline is right, given the correct game plan, anything is possible.
"It was a shock for people, but Celtic deserved it," he said of the win set up by goals from Victor Wanyama and Tony Watt.
"Barcelona had chances, but Celtic's defence was really good."
As it will need to be again tonight with van Dijk and Efe Ambrose - now a solid pairing after initial shaky moments - at the centre of it.
Van Dijk had his best game for the club in the San Siro, but this will be their stiffest test together, even without Lionel Messi there to torment them.